We study experimentally and numerically a (quasi) two dimensional colloidal suspension of selfpropelled spherical particles. The particles are carbon-coated Janus particles, which are propelled due to diffusiophoresis in a near-critical water-lutidine mixture. At low densities, we find that the driving stabilizes small clusters. At higher densities, the suspension undergoes a phase separation into large clusters and a dilute gas phase. The same qualitative behavior is observed in simulations of a minimal model for repulsive self-propelled particles lacking any alignment interactions. The observed behavior is rationalized in terms of a dynamical instability due to the self-trapping of self-propelled particles.PACS numbers: 82.70. Dd,64.60.Cn Following our physical intuition, "agitating" a system by, e.g., increasing the temperature also increases disorder. The most simple and paradigmatic example is the Ising model of interacting spins on a lattice, which, in two or more dimensions, displays a second-order phase transition from an ordered state to a disordered state as we increase the temperature [1]. Non-equilibrium driven systems, however, may defy our intuition and show the opposite behavior: increasing the noise strength leads to the emergence of an ordered state [2,3], for example the "freezing by heating" transition of oppositely driven particles in a narrow channel [4].One class of non-equilibrium systems that currently receives considerable attention are self-propelled, or "active", particles [5][6][7][8][9][10][11][12][13]. These are model systems for "living active matter" ranging from microtubules [14] to dense bacterial solutions [15][16][17] to flocks of birds [18]. A common feature of many of these models is that the particle orientations align, which leads to a multitude of collective phenomena such as swarming [19] and even micro-bacterial turbulence [20]. This alignment interaction can be either explicit (Vicsek-type models [21]) or indirect. For example, in dense granular systems of rods [22] and disks [23], the combination of hardcore repulsion and propulsion implies an effective alignment. Somewhat surprisingly, recently it has been found that also self-propelled suspensions lacking any alignment mechanism are able to show collective behavior. Specifically, simulations of a minimal model for a suspension of repulsive disks below the freezing transition [24] show phase separation into a dense large cluster and a dilute gas phase [25,26]. Phase separation due to a densitydependent mobility has been discussed theoretically in the context of run-and-tumble bacteria [27], and a link has been made recently to self-propelled Brownian particles [28].Experimentally, active clustering of spherical colloidal particles has been observed for sedimenting, platinumcoated gold particles [10] and colloidal particles with an embedded hematite cube [13], where platinum and hematite act as catalysts for the decomposition of water peroxide. In both studies, aggregation is attributed to attractive forces. In thi...

Active Brownian particles are capable of taking up energy from their environment and converting it into directed motion; examples range from chemotactic cells and bacteria to artificial micro-swimmers. We have recently demonstrated that Janus particles, i.e. gold-capped colloidal spheres, suspended in a critical binary liquid mixture perform active Brownian motion when illuminated by light. In this paper, we investigate in more detail their swimming mechanism, leading to active Brownian motion. We show that the illumination-borne heating induces a local asymmetric demixing of the binary mixture, generating a spatial chemical concentration gradient which is responsible for the particle's self-diffusiophoretic motion. We study this effect as a function of the functionalization of the gold cap, the particle size and the illumination intensity: the functionalization determines what component of the binary mixture is preferentially adsorbed at the cap and the swimming direction (towards or away from the cap); the particle size determines the rotational diffusion and, therefore, the random reorientation of the particle; and the intensity tunes the strength of the heating and, therefore, of the motion. Finally, we harness this dependence of the swimming strength on the illumination intensity to investigate the behavior of a micro-swimmer in a spatial light gradient, where its swimming properties are space-dependent.

Micron-sized self-propelled (active) particles can be considered as model systems for characterizing more complex biological organisms like swimming bacteria or motile cells. We produce asymmetric microswimmers by soft lithography and study their circular motion on a substrate and near channel boundaries. Our experimental observations are in full agreement with a theory of Brownian dynamics for asymmetric self-propelled particles, which couples their translational and orientational motion. [9] driving forces lead to active motion of micron-sized objects. So far, most studies have concentrated on spherical or rod-like microswimmers whose dynamics is well described by a persistent random walk with a transition from a short-time ballistic to a long-time diffusive behavior [10]. Such simple rotationally symmetric shapes, however, usually provide only a crude approximation for selfpropelling microorganisms, which are often asymmetric around their propulsion axis. Then, generically, a torque is induced that significantly perturbs the swimming path and results in a characteristic circular motion.In this Letter, we experimentally and theoretically study the motion of asymmetric self-propelled particles in a viscous liquid. We observe a pronounced circular motion whose curvature radius is independent of the propulsion strength but only depends on the shape of the swimmer. Based on the shape-dependent particle mobility matrix, we propose two coupled Langevin equations for the translational and rotational motion of the particles under an intrinsic force, which dictates the swimming velocity. The anisotropic particle shape then generates an additional velocity-dependent torque, in agreement with our measurements. Furthermore, we also investigate the motion of asymmetric particles in lateral confinement. In agreement with theoretical predictions we find either a stable sliding along the wall or a reflection, depending on the contact angle.Asymmetric L-shaped swimmers with arm lengths of 9 and 6 µm were fabricated from photoresist SU-8 by photolithography [11]. In short, a 2.5 µm thick layer of SU-8 is spin coated onto a silicon wafer, soft-baked for 80 s at 95• C and then exposed to ultraviolet light through a photo mask. After a post-exposure bake at 95• C for 140 s the entire wafer with the attached particles is coated with a 20 nm thick Au layer by thermal evaporation. When the wafer is tilted to approximately 90• relative to the evaporation source, the Au is selectively deposited at the front side of the short arms as schematically shown in Figs. 1(a),(b). Finally, the coated particles are released from the wafer by an ultrasonic bath treatment. A small amount of L-shaped particles is suspended in a homogeneous mixture of water and 2,6-lutidine at critical concentration (28.6 mass percent of lutidine), which is kept several degrees below its lower critical point (T C = 34.1• C) [12]. To confine the particle's motion to two dimensions, the suspension is contained in a sealed sample cell with 7 µm height. The particles ar...

Many motile microorganisms adjust their swimming motion relative to the gravitational field and thus counteract sedimentation to the ground. This gravitactic behaviour is often the result of an inhomogeneous mass distribution, which aligns the microorganism similar to a buoy. However, it has been suggested that gravitaxis can also result from a geometric fore-rear asymmetry, typical for many self-propelling organisms. Despite several attempts, no conclusive evidence for such an asymmetry-induced gravitactic motion exists. Here, we study the motion of asymmetric self-propelled colloidal particles which have a homogeneous mass density and a well-defined shape. In experiments and by theoretical modelling, we demonstrate that a shape anisotropy alone is sufficient to induce gravitactic motion with either preferential upward or downward swimming. In addition, also trochoid-like trajectories transversal to the direction of gravity are observed.

We demonstrate with experiments and numerical simulations that the structure and dynamics of a suspension of passive particles is strongly altered by adding a very small (o1%) number of active particles. With increasing passive particle density, we observe first the formation of dynamic clusters comprised of passive particles being surrounded by active particles, then the merging and compression of these clusters, and eventually the local melting of crystalline regions by enclosed active particles.